Fluid catalytic cracking (FCC) is primarily used to convert high boiling, high molecular weight hydrocarbons into lower boiling, lower molecular weight compounds. The lower molecular weight compounds include gasoline, olefinic compounds, liquid petroleum gas (LPG), diesel fuel, etc. An FCC unit typically uses a catalyst that is repeatedly deactivated and regenerated in a riser and a regenerator, respectively. Air is used to combust the coke from the deactivated catalyst in the regeneration process, and produces combustion gases such as carbon dioxide and water. Partial combustions gases like carbon monoxide may also be produced. Many FCC units use the energy generated from burning the coke from the catalyst to drive the endothermic reaction in the riser.
A residue fluid catalyst cracking (RFCC) unit is used to process residue feedstocks that tend to be higher molecular weight compounds than the feedstocks typically processed in FCC units. The residue typically produces more coke on the catalyst than vacuum gas oil or other typical FCC feedstocks, so more energy is produced when the coke is combusted. Many RFCC units include a two stage regenerator, where some of the coke is combusted from the catalyst in a first stage regenerator, and the remaining coke is combusted in a second stage regenerator. Some or all of the catalyst is cooled between the first and second stage regenerators to control the catalyst temperature, and thereby to maintain the energy balance between the regenerator and the riser.
A second flue gas from the second stage regenerator is vented into the first stage regenerator to provide some of the oxygen used in combustion, and additional air or other gases provide the remainder of the oxygen. The first stage regenerator often has a larger diameter than the second stage regenerator, and the first stage regenerator is positioned directly on top of a dome-shaped top of the second stage regenerator so some of the first stage regenerator is outside of the area directly above the top. In many traditional RFCC units the second flue gas is vented directly over the top of the second stage regenerator, and this extra gas increases the superficial gas velocity in the area over the top but not in the areas that are not over the top. The superficial gas velocity in the area directly over the top has been found to be up to about 2 to 3 times the superficial gas velocity in the area that is not directly over the top.
The catalyst is fluidized in the first and second stage regenerators, and catalyst loading in the flue gas increases exponentially with the superficial gas velocity. Catalyst is separated from a first flue gas in the first stage regenerator using cyclones, where the first flue gas is the total flue gas from the first and second stage regenerators. Catalyst in the flue gas is abrasive, and high catalyst loading is known to abrade equipment exposed to it. The higher superficial gas velocity in the area directly over the top has been found to entrain more catalyst and erode the cyclones and other equipment much more rapidly than in the area of the first stage regenerator that is not directly over the top.
Accordingly, it is desirable to provide catalyst regenerators and methods for regenerating catalyst that limit the abrasion of equipment in the first stage regenerator that is directly over the second stage regenerator. In addition, it is desirable to provide catalyst regenerators and methods for regenerating catalysts that reduce the superficial gas velocity directly over the second stage regenerator within the first stage regenerator. Furthermore, other desirable features and characteristics of the present embodiment will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawing and this background.